get_memory_usage() doctest failure

[jdemeyer]

Justin reports on sage-release:

Built from scratch on Mac OS X, 10.6.8 (Dual 6-Core Xeon) w/o problems.  Testing (ptestlong) yielded one failure:
----------------------------------------------------------------------
The temporary doctesting directory
   /Users/justin/.sage/tmp/Zippo.local-33453
was not removed: it is not empty, presumably because doctests
failed or doctesting was interrupted.

----------------------------------------------------------------------

The following tests failed:

	sage -t  -long -force_lib devel/sage/sage/misc/getusage.py # 1 doctests failed
----------------------------------------------------------------------
**********************************************************************
File "/Users/Sage/sage-4.7.2.rc1/devel/sage-main/sage/misc/getusage.py", line 93
:
    sage: get_memory_usage(t)          # amount of memory more than when we defined t.
Expected:
    0.0
Got:
    0.25
**********************************************************************

[leif]

EXAMPLES: should now be EXAMPLES:: (i.e., with a double-colon).

The program names (prstat and top etc.) should be typeset in monospaced / typewriter font, i.e. for example ``prstat`` instead of 'prstat'.

[leif]

Ok, new patch looks good (although I'd also use ``t`` instead of `t`, since it is a Python identifier rather than some mathematical variable or symbol), but Justin should test it and give it positive review.

[leif]

P.S.: FWIW, applies to rc0 and passes tests on Ubuntu 10.04.3 x86_64.

[justin]

I can't now reproduce the doctest failure; maybe cosmic rays are to blame.

In any case, the doctests all pass with the patch applied; the changes seem reasonable.